The present invention relates to a method and apparatus for monitoring radio signal strengths for a plurality of channels in a wireless communication system. In particular, but not exclusively, the present invention relates to such monitoring in a DECT-compliant communications system.
DECT is an abbreviation for Digital Enhanced Cordless Telecommunications to which the European Telecommunication Standard ETS 300 175 relates. Whilst the present invention has particular relevance to DECT-compliant communication systems, and is described hereinafter in relation to such systems, the skilled reader will recognize, however, that the method and apparatus described herein are more generally applicable and compliance with DECT standards should not be assumed unless explicitly stated.
A typical DECT compliant communications system includes at least one Portable Part (PP) which communicates over a wireless link with a radio fixed part (RFP) of a DECT Fixed Part (FP). The fixed part will be connected to other fixed equipment such as a PABX or a PSTN. The FP often includes more than one RFP, with each RFP serving to provide radio coverage of a particular area. The use of multiple RFPs allows large areas of radio coverage and a PP may move from one area covered by one RFP to an area served by another RFP with the same communication facilities being available. DECT PP""s commonly take the form of mobile handsets and depending on the implementation of the system, a large number of portable parts may be supported.
Communication between a fixed part and a portable part is provided by so-called xe2x80x98bearersxe2x80x99 which are established on one or more DECT physical channels (channel). Each channel is created by transmitting in one particular slot on one particular radio frequency (RF) channel in successive time division multiple access (TDMA) frames.
According to the DECT standards, when a portable part is activated it is the responsibility of the PP to perform an operation to detect the presence of any nearby RFP""s that are in range. This is made possible in a DECT-compliant system because each RFP broadcasts continuously on at least one bearer. Every bearer of a RFP broadcast carries Q-channel information (system information) and N-channel information (identity information). This information is used by the PP to determine if the RFP is one it has access rights to. If the RFP is not involved in supporting a traffic connection with a PP then the N-channel and Q-channel information is broadcast on a so called dummy bearer. If the RFP is involved in supporting one or more traffic connection with one or more PP, the Q-channel and N-channel information is present on each traffic bearer. A RFP supporting at least one traffic connection may drop the dummy bearer, although the broadcast of the N and Q-channel information on the dummy bearer must be restored should all traffic connections with that RFP cease . It is stated for clarity that the N-channel and the Q-channel are logical channels, not DECT physical channels.
Another task of the PP is to identify the RFP transmissions that are received at the PP with the highest detected radio signal strength. The RFP producing these transmissions is also identified and is designated as the strongest RFP. The strongest RFP will normally be the RFP that is closest to the PP and wherever possible communications between the PP and FP will be via the strongest RFP. Useful communication between the PP and the FP may only take place via the chosen RFP if the portable part has access rights to the FP associated RFP.
One procedure for selecting the strongest RFP is for the PP to monitor DECT system channels and build up a list of the channels that have the highest detected radio strength. The channels are included in the list in order of detected radio signal strength, with the channel having the highest detected radio signal strength appearing first.
Once the list has been produced the PP attempts to set up a synchronisation bearer on the channel listed as having the highest radio signal strength. This bearer set up operation is necessary to determine if the detected radio signal strength results from transmissions of a RFP of interest; that is an RFP of a required FP for which access rights will be granted to the PP. Indeed this step is also necessary because the source of the detected radio signals may originate from transmissions of an incompatible system or merely be due to noise. If it can be established from the N-channel and Q-channel information that the transmissions are associated with the required FP, further procedures are conducted allowing the PP to synchronise and lock with the FP and be in a state ready to set up and release traffic bearers in the interest of exchanging traffic (for example voice calls).
If the transmissions are not associated with the required FP the synchronisation bearer is released (if the set up attempt was successful) and the PP attempts to set up a synchronisation bearer on the channel listed as having the next highest detected radio signal strength. Again the PP carries out the steps necessary to determine if this channel carries transmissions from the required FP. This process is repeated until the PP finds a transmission that allows it to synchronise and lock with the required FP. This procedure, however, is not ideal as limits on list size may result in the list not including any entries that are associated with the required FP, as will be described in more detail in relation to FIG. 1 below.
An alternative procedure for selecting the strongest RFP is where the RFP attempts to set up a synchronisation bearer on each DECT channel in turn irrespective of the detected radio signal strength on the channel. This is repeated for each channel until the PP detects transmissions originating from an RFP of an FP it has access rights to. Once such transmissions have been found, the PP must continue searching each DECT channel until it locates a channel carrying transmissions of the same FP but with the highest detected radio signal strength. Such transmissions will generally originate from the closest RFP of the FP of interest and the PP may then conduct further procedures to synchronise and lock with the RFP and be in a state ready to set up and release bearers in the interest of exchanging traffic (for example voice calls).
There are also disadvantages to employing this alternative procedure, including the amount of time required for the PP to reach the state where it is ready to set up and receive traffic bearers, because an attempt to set up the synchronisation bearer would need to be made on every possible DECT channel to guarantee that the required FP and strongest RFP have been identified.
The portable part is also required to keep an ordered list of the DECT channels that have the least activity in terms of the radio signal strength. These so called quietest channels are the ones that will be chosen for supporting traffic bearers when the PP sets up a connection with the FP.
It is an object of the present invention to provide a method and apparatus for maintaining a record of radio signal strength for a plurality of channels in a wireless communication system, which may be used to facilitate the reliable identification of communication system components without the use of large amounts of memory.
In accordance with one aspect of the present invention there is provided apparatus for maintaining a record of radio signal strength for a plurality of channels in a wireless communication system, the apparatus having:
measuring means for detecting the radio signal strength on each channel; and
storage means for storing information relating to the detected signal strengths,
wherein the information is stored in the storage means in the form of a value for each channel, each value being representative of the respective detected signal strength on a channel, and the storage means includes an array, with the values being stored in positions in the array such that each position in the array is assigned to a particular channel.
The identity of each channel for which signal strength information is stored can be determined from the storage position in the array because the channel allocation for the various array positions is known. Advantageously, the amount of memory required for storing information relating to a channel can be reduced because there is only the requirement to store information relating to the signal strength of that channel without the need to record information about the channel identity. Another advantage is that because each channel value is recorded in its assigned position, the channels do not need to be sorted in order of detected radio signal strength thus eliminating the need to allocate processing resources to such a task.
The reduction in the amount of memory required for storing information about a given channel makes the storage of information about all DECT channels feasible since a relatively modest amount of storage capacity will be required. This avoids the disadvantages that may result when using a non exhaustive list. Furthermore since the array may contain signal strength information for all DECT channels, the array contains values representing the highest detected signal strengths and the lowest detected signal strengths. Consequently a single array may replace the conventional ordered list of channels having the highest detected radio signal strength and the ordered list of channels having the lowest detected radio signal strength. Preferably (although not exclusively), the array is a two dimensional array. Advantageously the value stored in each position of the array may occupy no more than one byte of storage.
In accordance with another aspect of the present invention there is provided a method of maintaining a record of radio signal strength for a plurality of channels in a wireless communication system, said method comprising the steps of:
detecting the radio signal strength on each channel; and
storing information relating to the detected signal strengths,
wherein the step of storing information is performed by storing a value for each channel that is representative of the detected signal strength on that channel, the values being stored in positions in an array such that each array position is assigned to a particular channel.
Furthermore, if the wireless communication system of the method is a DECT-compliant communication system, the method may further comprise the steps of:
scanning the array to identify the value representing the highest detected signal strength,
setting up a synchronisation bearer on the channel identified as having the value representing the highest detected signal strength; and
determining if the measured signal strength arises from transmissions originating from a radio fixed part of a required fixed part, and if the measured signal strength is determined to not arise from such transmissions, releasing the synchronisation bearer and performing the scanning, setting up and determining steps one or more further times only for one or more channels identified as having a value representing one of the next highest detected signal strengths.
Furthermore, if the wireless communication system of the method is a DECT-compliant communication system the method may further comprise the steps of:
scanning the array to identify the values representing the lowest detected signal strengths and selecting one or more of the associated channels for establishing one or more bearer.
Other aspects and optional features of the present invention appear in the appended claims, to which reference should now be made and the disclosure of which is incorporated herein by reference.